1. Field of the Invention
The present invention relates to a hard disk drive (HDD), and in particular, to a technique for reading an information storing sector on each track of an information recording magnetic disk.
2. Description of the Related Art
A hard disk drive has, for example, two disks and four heads, each head corresponding to one surface of the disks. For reading a preamplifier preamplifies a signal picked up by one of the heads and supplies an analog read signal to a read/write channel circuit while for writing, the preamplifier supplies a write current representative of encoded write data received from the read/write channel circuit to a corresponding head to thereby write the write data on the disk. The read/write channel circuit detects data pulses from the read signal received from the preamplifier and decodes the detected data pulses and supplies them to a disk data controller, or decodes the write data received from the disk data controller and supplies the decoded write data to the preamplifier.
The disk data controller includes a sequencer for implementing an operation according to a program downloaded from a microcontroller and writes data received from a host computer on the disk via the read/write channel circuit and the preamplifier, or reads data from the disk via the preamplifier and the read/write channel circuit and sends the read data to the host computer. The disk data controller also interfaces communication between the host computer and the microcontroller, and temporarily stores data transmitted between the host computer and the microcontroller and between the host computer and the read/write channel circuit.
The microcontroller controls the disk data controller in response to a read/write command received from the host computer and the read/write channel circuit. The microcontroller controls the disk data controller in response to a read or write command received from the host computer and also controls track seek and track following. A read only memory stores operation programs and various designated values of the microcontroller. The microcontroller supplies a control value for controlling rotation of the disks to the spindle motor driver to thereby drive the spindle motor driver and rotate the disks. A disk signal controller generates various timing signals needed for reading and writing under the control of the microcontroller, decodes servo information, and supplies the decoded servo information to the microcontroller.
On a hard disk having a constant density recording data format, a complete data sector and a split data sector are disposed between servo fields in a track.
Generally, an information recording area of a CDR disk is split into a plurality of zones having uniform recording densities and disposed radially from the center of the disk. The tracks in each split zone have different data sectors. That is, a track located in an outer circumferential zone has more data sectors than that in an inner circumferential zone. A data sector is a unit area for accessing data on a disk by a magnetic disk recording device, and has the same size, for example, 512 bytes, regardless of positions on the disk. In an embedded sector servo, which is one of the schemes for providing head position information to the magnetic disk recording device, one data sector can be split into two segements depending on zones on the disk, and servo information areas and data areas are alternately arranged along the circumferential direction in each track. A servo information area is a servo field for recording servo information, and a data area is used for recording actual data and provides a data sector.
Each data sector is divided into an identification field and a data field. Header information for identifying a corresponding data sector is recorded in the identification field and actual digital data is recorded in the data field following the identification field.
The data field is composed of a VFO (variable frequency oscillator) being a data preamble, a sync pattern being a data address mark, actual data, ECC (error correction code), and PAD being a data postamble. The data preamble VFO is disposed between a postamble of the identification field and a sync pattern and is a synchronization signal for clock synchronization when reading the data field. The sync pattern provides synchronization necessary for reading the data with the magnetic disk recording device by notifying as to the start of the data. The data is digital data actually stored on the disk. The error correction code is an error detection code for detecting and correcting errors in the data address mark and the data. The postamble PAD provides a timing margin needed after reading the data.
When sector pulses are generated in a sector pulse generating circuit of the hard disk drive, a sequencer of the disk data controller senses the sector pulses, detects the starting position of a data sector, and sets a read gate signal to a high level so as to start reading the data field.
However, the abovenoted read operation has a problem as follows. The read gate signal is generated at a predetermined time, determined according to a microprogram, after the sector pulses are generated. Then, an automatic gain control level and phase of a phase locked loop circuit are simultaneously adjusted according to a detected VFO signal. The automatic gain control and phase locked loop are completely adjusted by the time the sync pattern is detected. An automatic gain control circuit, provided in the read/write channel circuit, automatically controls the gain of a signal preamplified in the preamplifier, amplifies the gain controlled signal, and outputs the amplified signal. There is no problem when the VFO signal is normal in amplitude and wave-shape. On the other hand, the VFO signal may be defective at a rising edge of a read gate signal or the VFO signal may be inaccurately written due to jitter of the spindle motor during writing. With defects in the VFO signal, the automatic gain control level and the phase locked loop phase are not properly adjusted.
The patent to Okada et al., U.S. Pat. No. 5,642,244, entitled Method And Apparatus For Switching Between Data And Servo Modes, discloses features in common with the present invention but does not teach or suggest the specifically recited technique for reading an information recording field in accordance with the present invention.